So here's an update, not sure it's what anybody wants to hear, but here goes.
I replaced my new 5700 xt on Monday because it was showing faults in both Windows and macOS. Aside from these watchdog / windowserver panics on idle about once every day or 2, I was seeing the displays glitching (both at the same time, with the same pattern; looked like a sync issue) followed shortly after by both screens going black and seemingly halting everything else (any playing sounds stopped). Also about once every day or 2.
I posted my comment about Big Sur's drivers shortly after replacing the faulty gfx card. I went about a day in there without any issues, but then found I needed to boot Catalina again because of some work software that wasn't quite ready for Big Sur yet. I decided to stay in Catalina, since if I replaced the gfx card *and* moved OS, it would be hard to know which change affected my issues (if indeed there was any difference in behaviour). I haven't seen one of these panics since.
When I was seeing this issue on my 2019 Vega MBP, it would happen on average between 1-2 times per week. I think my record uptime was around 7-8 days with both screens attached. So for me, having gone 4-5 days without one of these panics proves nothing. It could still come back.
But, the fact that this panic was happening daily using the faulty graphics card does strongly suggest that faulty hardware is one potential cause of this crash. This doesn't imply there aren't other causes - in fact there are already several different routes to this crash described in this thread, which I suspect must have different underlying causes. This panic only occurred on my MBP when it was idle and close to transitioning to the sleep state. Playing a video on loop would prevent this from happening. Some people here are finding that playing a video causes the panic for them... so, yeah it's hard to really conclude anything other than this is a mess.
What follows may be an unrelated aside, but I'm posting it here for completeness.
This morning I woke up to the Hack refusing to recognise that one of my monitors (which are identical) was capable of 4k@60Hz. No amount of rebooting or reconnecting monitors changed that. I had to remove the com.apple.windowserver.plist file from /Library/Preferences (losing all other display settings) in order to regain the full capability of my screen. I also saw this exact behaviour using the aforementioned MBP and reported it to Apple. I have no idea of the cause, and Apple haven't responded to the bug report I filed.
If anything new and interesting happens, I'll post it here.
